During the cell handover in the LTE system, the idea of User Equipment (UE) assistant to network control, i.e., the steps of “measure-report-decide-execute”, is still adopted. When the Source Evolved Node B (S_eNB) decides that the UE shall handover to the Target Evolved Node B (T_eNB) according to measurement reports from the UE and the T_eNB, the S_eNB exchanges information with the T_eNB directly via an X2 interface, to complete resource preparation in the target cell; then, the S_eNB instructs the UE to handover to the target cell; after a successful handover, the T_eNB informs the S_eNB to release radio resource of the original cell. In addition, the S_eNB transmits data that is not transmitted yet to the T_eNB and updates relation of nodes between a user plane and a control plane.
Referring to FIG. 1, a flowchart of cell handover in the LTE system in the prior art mainly includes:
[1] The S_eNB configures the UE with a measurement procedure;
[2] The UE sends a measurement report to the S_eNB;
[3] The S_eNB makes a UE handover decision according to the measurement report and Radio Resource Management (RRM) information;
[4] The S_eNB sends a handover request to the T_eNB;
[5] On receiving the handover request from the S_eNB, the T_eNB performs an admission control procedure according to the received E-RAB Quality of Service (QoS) information;
[6] The T_eNB sends a Handover Request Acknowledge message to the S_eNB;
[7] The S_eNB sends a Handover Command message to the UE, to instruct the UE to perform a handover;
[8] The UE sends a Synchronization message to the T_eNB;
[9] The T_eNB feeds a Synchronization Reply message back to the UE, the Synchronization Reply message including UL resource allocation information, a time advance, etc.;
[10] The UE sends a Handover Complete Acknowledge message to the T_eNB after the UE accesses the target cell successfully.
On the user plane, in order to avoid loss of uplink data from the UE and to maintain ordered transmission of the uplink data during the handover, an uplink data forwarding solution is adopted, which is performed between step [7] and step [8] of FIG. 1 (as illustrated by dotted lines). Generally, the data forwarding refers to that: the S_eNB forwards received disordered Packet Data Convergence Protocol-Service Data Units (PDCP-SDUs) to the T_eNB, and the PDCP-SDUs are attached with Sequence Numbers (SNs), then the T_eNB sends the received PDCP-SDUs to the Service Gateway (S-GW). During the handover, ordered transmission of upper layer PDUs is based on continuous PDCP-SNs and a re-order function provided by the PDCP layer. In the uplink, the re-order function of the PDCP layer of the T_eNB ensures the ordered transmission of the uplink PDCP-SDUs.
In the present LTE system, the air interface is based on the Automatic Repeat Request/Hybrid Automatic Repeat Request (ARQ/HARQ) mechanism. The S_eNB feeds ACKs back to the UE on successfully receiving ordered PDCP-SDUs. The UE re-transmits to the T_eNB those PDCP-SDUs whose ACKs are not received at the UE. It is not difficult to understand that the UE does not receive any ACK to a PDCP in two cases: in the first case, the S_eNB does not receive the PDCPs due to reasons such as network, etc., and does not feed ACKs back to the UE accordingly; in the second case, the S_eNB has received ordered PDCPs successfully, however, ACKs are not fed back to the UE successfully due to network reasons. The above second case may result in insignificant re-transmission from the UE to the T_eNB and a waste of precious radio resource. Examples are described hereinafter with reference to the two cases.
FIG. 2 illustrates a fact in which re-transmission of PDCP-SDUs from the UE results in an insignificant waste. The S_eNB receives data 1, 2 and 3 from the UE successfully and transmits the data 1, 2 and 3 to the S-GW via an S1 interface. It is assumed that the UE receives only an ACK to the data 1 fed back from the network side and does not receive ACKs to the data 2 and 3. In this case, the UE continues to send the data 2 and 3 to the T_eNB, the T_eNB receives the data 2 and 3 and sends the re-transmitted data 2 and 3 to the S-GW, which results in reception of repeated data in the Evolved Packet Core (EPC) system and a waste of precious radio resource at the air interface.
FIG. 3 illustrates another fact in which re-transmission of PDCP-SDUs from the UE results in an insignificant waste. The UE sends data a, b, c and d to the S_eNB, however, the data b fails during transmission and the S_eNB receives only the data a, c and d, and the data c and d becomes disordered. According to the conventional solution of data forwarding, the S_eNB sends the ordered data a to the S-GW, buffers the disordered data c and d in a buffer, and then sends the data c and d to the T_eNB which delivers the data c and d to the S-GW. It is assumed that the UE does not receive any ACK to the data c and d. In this case, the UE receives only an ACK to the data a, and re-transmits the data b, c and d to the T_eNB because the UE does not receive any ACK to the data b, c and d. Apparently, the data c and d is re-transmitted. On one hand, the S_eNB sends the buffered data c and d to the T_eNB; on the other hand, the UE re-transmits the data c and d to the T_eNB due to no reception of any ACK, thereby resulting in a waste of resource.